Objectives: 1. To analyze the relative roles of the mammary gland, kidney, and liver in the metabolic clearance of immunoactive prolactin from the plasma of rats during late pregnancy and throughout lactation by the constant infusion to steady state method. 2. To use metabolic clearance rates of prolactin plus endogenous prolactin levels, as measured by radioimmunoassay, to determine the secretion rate of prolactin into the circulation during late pregnancy and during lactation. 3. To determine the doses of intravenously injected rat prolactin required to stimulate minimal, i.e., threshold, as well as maximal milk secretion in the freely moving conscious rat at various stages of lactation; also to determine the roles of STH and ACTH in modifying these milk secretory responses. 4. Correlate minimal and maximal milk secretory responses with circulating prolactin concentrations during various stages of lactation. 5. To investigate with sensitive intramammary pressure techniques whether estrogen and progesterone exert either facilitatory and/or inhibitory influences at the mammary gland level upon the contractile response of the mammary gland myoepithelium. 6. To analyze the role of the cerebral cortex and the sympathetic nervous system in the regulation of mammary contraction.